I usually prefer one that includes more of the body than just a guy's cock, an anonymous cock doesn't really give much of an idea of what someone's body is like, and I never find cock pics really erotic although I do like cock. We all like to communicate with someone we find attractive so I think it's best to be upfront and post as much of your body as you're able to (face in pic is always lovely but not many people are willing to post this type of photo), it's better to know before you start chatting or emailing someone that they're actually to your taste rather than to base an opinion that someone's going to have your ideal body on just their cock, then find out they're actually the opposite of what you like.